1. Field of the Invention
This invention relates, in general, to the field of semiconductor manufacture. In particular, it relates to monitoring the conditions of post chemical mechanical polishing processing.
2. Description of Related Art
Fabrication of semiconductor integrated circuits (IC) is a complicated multi-step process for creating microscopic structures with various electrical properties to form a connected set of devices. As the level of integration of ICs increases, the devices become smaller and more densely packed, requiring more levels of photolithography and more processing steps. As more layers are built up on the silicon wafer, problems caused by surface non-planarity become increasingly severe and can impact yield and chip performance. During the fabrication process, it may become necessary to remove excess material in a process referred to as planarization.
A common technique used to planarize the surface of a silicon wafer is chemical mechanical polishing (CMP). CMP involves the use of a polishing pad affixed to a circular polishing table and a holder to hold the wafer face down against the rotating pad. A slurry containing abrasive and chemical additives are dispensed onto the polishing pad.
The wafer and polishing pad rotate relative to each other. The rotating action along with the abrasive and chemical additives of the slurry results in a polishing action that removes material from the surface of the wafer. Protrusions on the surface erode more efficiently than recessed areas leading to a flattening or planarization of the wafer surface. Following CMP, the wafer must be cleaned of any CMP and slurry residue. Any residue remaining on the wafer can cause shorts to the IC devices.
A known method of post-CMP processing in the prior art utilizes brush cleaning technology such as the Dai Nippon Screen Model No. SP-W813-AS (DNS brush cleaner). The DNS brush cleaner cleans the wafer using a combination of rinsing, megasonic rinsing, and brush cleaning.
The wafers are loaded into a wet environment, usually water, then transported through a series of cleaning chambers for the brush cleaning cycle. The brush cleaning cycle involves rotating the wafer at high speed, about 1500 rpm, while a jet of deionized water is sprayed on the wafer to dislodge any loose debris from CMP, and the wafer is brushed with a foam brush.
During the brush cleaning cycle, the brush is first placed over the center of the wafer. The brush contacts the backside of the wafer, presses down on the wafer, and moves at a constant height and pressure to the periphery of the wafer in one stroke. The brush then retracts from the wafer and the whole cycle is repeated. The following chamber then brushes the top side of the wafer. After the brushing cycles, the wafer is deposited in the spin/rinse/dry chamber and unloaded dry.
The problem associated with the DNS brush cleaner is the inability of the machine to recognize whether the brush has fallen off and is not making contact with the wafer. A tool user cannot actually see that the brush has fallen off the brush holder since the cleaning process is within an enclosed chamber. Also problems arise when the brush's contact pressure is improperly set during maintenance set up or during the brush cleaning cycle. The silent failure of the brush to properly clean the wafer is difficult to detect until the next inspection point. At that time, several wafer lots have gone through the brush cleaner and would require reworking the lots, a costly solution.
U.S. Pat. No. 4,561,214 issued to Inoue on Dec. 31, 1985, discloses a motor driven abrading tool having a sensor to monitor electrical resistance between the contact surfaces of the tool and the object being abraded. As the contact resistance increases with the decrease of the urging pressure of the tool, the current from the electrical source is reduced and the voltage drops. A control circuit responds to the drop in voltage to control the rpm of the abrading tool motor or to control the torque on the tool to change the contact pressure of the tool on the object being abraded. This patent does not disclose or suggest monitoring the torque as a means to detect the presence of the abrading tool but rather as a means to adjust the contact pressure of the tool.
U.S. Pat. No. 4,757,566 issued to Field et al. on Jul. 19, 1988, discloses an automatic tool torque compensator for a surface maintenance machine. Once an operator of the machine determines what level of tool torque is desired, the tool torque compensator automatically maintains the tool torque at the desired setting although the surface maintained may vary in elevation or texture. The tool torque compensator achieves the torque compensation by using an electrical signal representative of current load for the tool motor and a signal representative of the desired level of tool torque and integrates the two signals to raise or lower the surface maintenance tools which change their pressure against the surface being maintained. However, this patent discloses adjusting the contact pressure of the tool on the surface being maintained and does not disclose or suggest using the torque value as an input to detecting the presence of the tool itself.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a post-CMP brush cleaner for semiconductor wafers having a means for ensuring proper wafer cleaning.
It is another object of the present invention to provide a method of ensuring proper wafer cleaning during post-CMP processing.
A further object of the invention is to provide a method of ensuring proper wafer cleaning during post-CMP processing utilizing a DNS brush cleaner.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.